1. Field of the Invention
The present invention relates to a length measuring apparatus employing a magnetostrictive line suitable for use in measurement of a gasoline level surface in a petrol tank of a petrol station, detection of a stroke of a piston rod assembled in a hydraulic cylinder, detection of a position of a movable portion of various kinds of industrial machines and so on.
2. Background of the Invention
A length measuring apparatus employing an ultrasonic magnetostrictive delay line whose length is extended or reduced by application of a magnetic field thereto includes a length measuring apparatus employing an ultrasonic magnetostrictive delay line proposed by the assignee of this application and disclosed in U.S. Pat. No. 5,583,433.
The length measuring apparatus employing the ultrasonic magnetostrictive delay line will hereinafter be described with reference to FIG. 1. As shown in FIG. 1, an ultrasonic magnetostrictive delay line 10 is provided. A magnet piece 11 is disposed away from the ultrasonic magnetostrictive delay line 10 by a predetermined distance so as to be movable in the length direction of the ultrasonic magnetostrictive delay line 10 and so as to be opposed thereto. A detection coil (receiving coil) 3 is wound around the ultrasonic magnetostrictive delay line 10 over the whole area of a length measuring range thereof. A wave transmitter 4 (having a drive coil) is disposed at one end of the ultrasonic magnetostrictive delay line 10. A drive pulse generator circuit 5 applies a predetermined drive pulse A to the wave transmitter 4. A detector 6 detects a detection pulse signal B as an induced voltage generated in the detection coil 3. An arithmetic circuit 7 calculates a delay time of a detection pulse relative to a drive pulse to thereby output a signal corresponding to the distance from an output terminal e thereof. A ultrasonic vibration C from the wave transmitter 4 is propagated in the length direction of the magnetostrictive delay line 10.
As shown in FIG. 7A, when the ultrasonic vibration C from the wave transmitter 4 is applied to the ultrasonic magnetostrictive delay line 10 based on the drive pulse A from the drive pulse generator circuit 5, the ultrasonic vibration is propagated in the ultrasonic magnetostrictive delay line 10. A detection pulse signal B shown in FIG. 7B is a pulse current generated at the detection coil 3. In FIGS. 7A and 7B, a time T.sub.1 is a time required for the ultrasonic vibration to be reciprocated in the above ultrasonic magnetostrictive delay line 10, and a time T.sub.2 is a time from a timing of the drive pulse A to a timing of the detection pulse B.
When the drive pulse A from the drive pulse generator circuit 5 is applied to a drive coil of the wave transmitter 4, a magnetic field is generated in the drive coil. The magnetic field generated in the drive coil produces a magnetostriction effect to the ultrasonic magnetostrictive delay line 10, and the ultrasonic vibration C is propagated in the ultrasonic magnetostrictive delay line 10.
At this time, although a pulse voltage is not produced when only the detection coil 3 is provided, if the magnetic piece 2 is disposed at a position close to the ultrasonic magnetostrictive delay line 10, then the ultrasonic magnetostrictive delay line 10 is partially magnetized. As a result, when the ultrasonic vibration c passes by the magnetized portion of the ultrasonic magnetostrictive delay line 10, the detection pulse B resulting from an induced voltage is generated in the detection coil 3 due to a so-called reverse magnetostrictive effect in which magnetization of the ultrasonic magnetostrictive delay line 10 is changed. The detection pulse B is generated twice due to reflection thereof at an open end. This first detection pulse B is detected by the detection circuit 6.
If the magnet piece 2 is moved along the ultrasonic magnetostrictive delay line 10, the time T.sub.2 from the generation of the drive pulse A to the generation of the detection pulse B in the detection coil 3 is changed in proportion to a distance by which the magnet piece 2 is moved. The drive pulse A from the drive pulse generator circuit 5 and the detection pulse B from the detection circuit 6 are supplied to the arithmetic circuit 7. The arithmetic circuit 7 calculates a time from the generation of the drive pulse A to the detection of the detection pulse B and then outputs the output signal corresponding to the distance from the output terminal e thereof.
The ultrasonic vibration C transmitted from the wave transmitter 4 is propagated in the ultrasonic magnetostrictive delay line 10 and repeatedly reflected by one end portion thereof and the other end thereof, finally gradually being attenuated to disappear.
Since a length L of the ultrasonic magnetostrictive delay line 10 is previously known, the length measuring apparatus is arranged by setting a period T.sub.1 of generation of the drive pulse A in the drive pulse generator circuit 5 to the same as a period in which the ultrasonic vibration C is reciprocated in the ultrasonic magnetostrictive delay line 10 having the length L and then returned to the wave transmitter 4.
A condition that the drive pulse generator circuit 5 generates the drive pulse A is set such that when the ultrasonic vibration C travels by the length 2L and is reflected at an end portion, where the wave transmitter 4 is provided, of the ultrasonic magnetostrictive delay line 10, the wave transmitter 4 is driven to generate a new ultrasonic vibration so that its phase should be the same as a phase of the ultrasonic vibration C.
As described above, since the strong ultrasonic vibration C having a matched phase can be obtained by overlapping the ultrasonic vibration C and a new ultrasonic vibration and hence a detection wave having a large vibration with a satisfactory S/N ratio can be obtained, it is unnecessary to suppress a reflected wave by a non-reflection supporting member. Therefore, it is possible to downsize the whole length measuring apparatus to an extent of a space of the non-reflection supporting body.
However, since in the above length measuring apparatus employing the ultrasonic magnetostrictive delay line magnetization of the ultrasonic magnetostrictive delay line 10 carried out by the magnet piece 2 is recorded in the direction in which the magnet piece 2 is moved, hysteresis is produced. If a magnetostrictive delay line having no remanence is employed, no hysteresis is produced but the ultrasonic vibration becomes smaller, which makes it difficult to detect a position. As a result, disadvantageously the magnetostrictive delay line presenting large ultrasonic vibration and large remanence is inevitably employed.
If a distance between the magnet piece 2 and the ultrasonic magnetostrictive delay line 10 is changed, then a voltage induced in the detection coil 3 is increased or decreased, which prevents a stable detection of the position of the magnet piece 2.